The removal of molybdenum from industrial wastewater has long posed a difficult and expensive technological challenge. This is a particular issue in sectors such as the service and maintenance of transportation vehicles and conveyances, military hardware and construction equipment, in which lubricants such as molybdenum disulfide are commonly used, particularly on wheel and brake parts, aircraft engines and other aerospace and military applications. The cleaning of such equipment and associated parts/hardware generates an oily wastewater containing molybdenum, along with other metals which may leach from the metal parts surfaces, such as cadmium, chromium, nickel, copper, lead and zinc. One of the most significant challenges exists due to the high solubility of molybdenum disulfide, which does not respond to traditional alkaline precipitation processes.
Before such wastewater can be discharged for disposal, strict federal, state and local environmental requirements must be met. This requires treatment of the wastewater to lower the content of molybdenum and the other metals to stringent discharge limits, often to levels such as less than 1 part per million (ppm).
One of the treatment technologies which has been used for the removal of molybdenum from wastewater is ion exchange. However this method is expensive, as it involves high capital and operating costs. In addition, this method is not effective in the treatment of oily wastewaters such as those identified above. A second method which has been used for molybdenum removal is reverse osmosis, in which the wastewater is passed through a semi-permeable membrane to separate the metals components from the wastewater. However, this method also has high capital and operating costs. In addition, once the reverse osmosis process has been completed, the concentrate (the waste stream containing the removed contaminants) must then be treated and disposed at a significant cost.
The traditional methodology used for the removal of metals from wastewater is the precipitation of the metal ions, typically involving the increase in the pH of the wastewater to facilitate the precipitation. However, this method can be time consuming and inefficient. Certain metals, such as cadmium and chromium, may precipitate out of solution under alkaline conditions, while metals such as molybdenum will not precipitate as easily and will typically remain in solution at levels above environmental regulatory discharge limits.
U.S. Pat. No. 4,219,416 issued to Ramirez et al. discloses an existing method for removing molybdenum and tungsten from mining wastewater. This patent teaches that methods of precipitating or settling out these minerals are inefficient and discloses a method of removing them using gas flotation. A trivalent metal cation is added to the wastewater and its pH is lowered. After an initial treatment of electrolytically driven dissolved air flotation, a flocculating polymer is added and the wastewater is again subjected to dissolved air flotation. This system is costly, inefficient and time consuming.
U.S. Pat. No. 4,485,075 issued to Maurel discloses a method of extracting arsenic in various metals. Lime is added to raise the pH and cause various contaminants to precipitate. Because the process requires evaporation of effluent it is time consuming. Furthermore, it requires stoichiometric determinations and is therefore complicated.
U.S. Pat. No. 5,281,339 issued to Mainwaring et al. discloses a method of removing contaminants by forming a foam, drying it and removing it from the waste water. The foam is formed by adding the sulfates or hydroxides of either iron or aluminum and a strong surfactant to the effluent and mixing it. The foam is then pulled off the wastewater, dried and removed. Gas is applied to the wastewater to induce formation of the foam. It teaches that sedimentation is a slow, undesirable process.
Accordingly, there is a need for an improved method of the treatment of molybdenum-containing wastewater. The present invention provides a method whereby molybdenum, along with other metal ions, is readily precipitated and separated from the wastewater. Through the addition of a metal sulfate, such as alum, to the wastewater, an effective method of precipitation of molybdenum and other metals under acidic conditions is presented. Alum has traditionally not been used in the precipitation of molybdenum and has typically not been effective under acidic conditions. Due to the efficiency of this process, neither reverse osmosis, gas flotation, ion exchange nor any other unit operation is needed.